“Meitos are famous bladed weapons of high quality and superior craftsmanship. They are prized by many for their immense cutting power and exceptional durability that makes them far superior to ordinary weapons.”

“There are currently four known grades of Meito. Supreme Grade, of which there are 12; Great Grade, of which there are 21; Skillful Grade of which there are 50; and Grade, of which there is a currently unknown number.”

“There is a circumstance where, after countless battles, a sword's blade will permanently turn black, thus it will become what is known as a "Black Blade" (黒刀, Kokutō?). Black Blades are stronger than ordinary swords and are well known for their impressive resilience; a dinosaur could step on one and it wouldn’t bend at all. It is said that should a swordsman be able to turn their blade into a Black Blade, its grade would increase by a rank.[1]”

Black blade are definitely stronger..it was confirmed..it is just how to permanently turn it into Black Blade is another mystery..only 2 sword's God who has it anyway, maybe deeper understanding about sword make it black??

It is stronger. Even much stronger

What Stone Pirates wrote, but it seems like imbuing weapons with haki might be the key. The blades turn black once imbued and I guess it just sticks and changes the blade’s durability and destructive power. Once changed they raise in level. Enma is one of the 21, but might increase in rank if it becomes a black blade.

When a blade is imbued with COA, it becomes black. However, a black blade is one that is permanently imbued with Haki, and remains black even when the owner isn't there or Haki isn't being channelled into it. There are only two black blades in the entire series: Shusui and Yoru. However, it is unknown how to turn a normal blade into a Black Blade.

I think it happens when you imbued your haki to a regular blade. and if your haki is strong enough it will permanently imbued to the regular blade turning it into a black blade